1. Field of the Invention
The present invention is generally in the field of electrical circuits. More specifically, the present invention is in the field of oscillators and tuning circuits.
2. Background Art
Voltage controlled oscillators (VCOs), such as differential VCOs, can be utilized to provide operating frequencies for communications devices, such as cell phones, as well as other types of electronic devices. A VCO, such as a differential VCO, generally includes a transconductance (Gm) stage coupled to a tank circuit, which can include an inductor and a varactor circuit for tuning the output frequency of the VCO. For a VCO, such as a differential VCO, performance can be increased by reducing phase noise, which is inversely proportional to the quality factor (Q) of the VCO tank circuit. Thus, the phase noise of the VCO can be reduced by increasing the Q of the inductor and/or increasing the Q of the varactor circuit. The Q of the inductor can be increased, for example, by utilizing multiple metal layers to form the inductor. However, it is also important to increase the Q of the varactor circuit, particularly for VCO operation in the gigahertz frequency range.
A conventional varactor circuit for a VCO, such as a differential VCO, can include two varactors, where each varactor can be coupled between a control voltage and one of the differential outputs of the VCO, and where each of the varactors can be a metal oxide semiconductor (MOS) varactor, such as an accumulation mode MOS (AMOS) varactor or an inversion mode MOS (IMOS) varactor. The Q of each varactor is inversely proportional to the capacitance of the varactor, which is proportional to the varactor gate area. Thus, the Q of each varactor in the conventional varactor circuit can be increased by reducing the capacitance of the varactor. However, the tuning gain of the VCO is proportional to the capacitance that the varactors present to the tank circuit. Thus, reducing the capacitance of the varactors to increase the Q of the conventional varactor circuit can reduce the VCO tuning gain, which is undesirable.